The increasing scarcity and the resulting increasing cost of fuel make it mandatory that today's burners operate at top efficiency. Further, environmental concerns require that the discharge of pollutants, and particularly of nitrogen oxides (hereinafter NOX), be minimized.
Amongst the measures to attain high efficiency, operating of the burner with the least amount of air is of utmost importance. This means that a burner should be operated with the least amount of excess air, that is the theoretically required amount of air to fully combust the fuel (hereinafter in the specification and the claims sometimes referred to as "stoichiometric air") plus the amount of excess or additional air that must be supplied to assure that all fuel will be combusted or oxidized. With today's best burner designs that typically means an amount of air only about 5% above the stoichiometric air.
To minimize the NOX emissions, more intricate measures must be taken. NOX is generated at high temperatures, that is temperatures in excess of about 3000.degree. F. Because of the greater affinity of oxygen (in the combustion air) to combustible materials in the fuel as compared to nitrogen, practically no NOX is generated while there is an oxygen deficiency, that is when the fuel is combusted in the presence of less than the theoretically required amount of air to fully combust the fuel (hereinafter in the specification and the claims also sometimes referred to as "off-stoichiometric air"). Further, practically no NOX forms at temperatures below about 3000.degree. F. Thus, as long as the fuel is combusted with off-stoichiometric air or the temperature is kept below 3000.degree. F. there are substantially no NOX emissions.
In large industrial furnaces such as boilers for electric generating plants, which employ multiple burners, these objectives are attained by controlling the air supply so that there is a "staged combustion", that is a combustion which, during a first stage, takes place in an oxygen deficient environment and, in a second stage, takes place at a relatively low temperature. Systems to accomplish this typically employ auxiliary air inlets spaced from and in relation to the burners so that the secondary combustion air, that is the air to bring about a complete combustion of the fuel, is introduced downstream of the burners. This approach has proved most successful.
However, small furnaces such as package boilers have only a single burner and a pre-assembled combustion chamber which does not permit an arrangement as disclosed in the preceding paragraph because of the size, shape, orientation and construction of the combustion chamber which typically is horizontal and has a tubular shape through which the flame extends. There is no space to arrange separate air inlets above the burner or in the side walls of the combustion chamber.
Consequently, prior art package boilers and furnaces employing only one burner either had to be operated at low temperatures to avoid the generation of NOX, which meant they had to be operated with large amounts of excess air which reduced their efficiency, or if high efficiency was most important, high levels of NOX emissions had to be accepted. Neither alternative is acceptable under today's economic conditions and concerns for the protection of the environment.
Further, when operating such a burner at the required relatively low temperature to prevent NOX emissions the boiler had to be de-rated, that is its steam producing capacity had to be lowered to prevent the formation of excessively long flames which would extend into the boiler convection section.